Before the fix:
```sh
$ grep Version result/lib/pkgconfig/librtlsdr.pc
Version: v0.8.x-xxx-xunknown
```
It causes problem while building projects that use librtlsdr:
```sh
-- Checking for module 'librtlsdr>=0.5.4'
-- Requested 'librtlsdr >= 0.5.4' but version of RTL-SDR Library is v0.8.x-xxx-xunknown
```
After the fix:
```sh
$ grep Version result/lib/pkgconfig/librtlsdr.pc
Version: 0.8.0
```
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